1. Field of the Invention
This invention relates to a screw type mechanical supercharger for use with a reciprocating internal combustion engine which is used, for example, for automobiles, marine vessels or general industries.
2. Description of the Prior Art
Such a screw type mechanical supercharger of the oil free type as shown in FIG. 3 is already known which is mechanically driven by way of a power transmitting mechanism including a gear, a belt or a chain by a crankshaft which is an output power shaft 10 of an engine (not shown).
Referring to FIG. 3, a body of the supercharger includes a pair of male and female screw rotors 11 and 12 held in a mutually meshing condition with each other with a small gap left therebetween without contacting with each other. The screw rotors 11 and 12 are connected to rotate in a synchronized relationship by a pair of synchronizing gears 13 and 14 which are mounted on ends of the rotor shafts and held in meshing engagement with each other.
A small gear 15 is mounted at the other end of the shaft of the male rotor 12 while a large gear 17 is mounted on an input power shaft 16 which is connected to be driven by the engine (not shown). The large gear 17 is held in meshing engagement with the small gear 15 so that the male rotor 12 may be driven by the engine (not shown). The female rotor 11 is thus driven in a synchronized relationship by the rotor 12 by way of the synchronizing gears 13 and 14.
A supercharger as described above commonly employs a combination of a male rotor having four or five teeth and a female rotor having six teeth.
With the supercharger described above, a driving force is transmitted via the input power shaft 16, large gear 17, small gear 15, male rotor 12, synchronizing gears 14 and 13 and female rotor 11 to rotate the two rotors 12 and 11, thereby to compress air sucked in from one side of the rotors 12 and 11 and forward or feed the compressed air from the other side of the rotors 12 and 11 into the engine.
It is a matter of importance to a supercharger applied, for example, to an automobile to minimize an increase in weight of the automobile or reduce the weight of the automobile and also to be of a small size because it is installed in an engine compartment.
Further, since an automobile is subjected to sudden acceleration or deceleration, follow-up is required for rotation of rotors of a supercharger. Consequently, a reduction in weight of the rotors is essential.
Meanwhile, although a supercharger is required to provide a compression pressure of about 2 kg.multidot.G/cm.sup.2 from the characteristic thereof, it is required, in the case of a large vehicle such as a truck, to provide an amount of supercharged air on the order of about 10 m.sup.3 /min.
While it may seem preferable to raise the rotational speed of an input power shaft from an engine in order to increase the amount of supercharged air, the rotational speed of the engine itself cannot be raised freely from the efficiency characteristic of the engine. Also with such a mechanical supercharger to which power is transmitted from an engine by means of a belt or the like as disclosed in applicants' U.S. Pat. No. 4,826,412earlier, there occurs a problem in that the rotational speed of the engine cannot be raised freely due to a problem with the life of the belt or slipping of the belt during high speed transmission or the like.